1. Field of the Invention
The present invention relates to a circuit for driving the organic light emitting diode (OLED) display panel and, more particularly, to a constant current driver with auto-clamped pre-charge function.
2. Description of Related Art
The organic light emitting diode (OLED) is known as an organic thin film semiconductor based light emitting device. Thus, a display panel can be provided by a two-dimensional array of OLEDs.
In general, an OLED panel may be driven by a constant voltage, which is deemed to be less energy consumed. However, because the cut-in voltages of the OLEDs on the display panel are not uniform, each OLED may de conducted in different voltage level, which results in that the emitted light is not even.
Furthermore, it is known that the light intensity of the OLED is proportional to the current generated by combining the electrons and holes at the junction area. This current is an exponential function of the junction voltage, so that it is very sensitive to the variance of the junction voltage. Hence, in order to achieve a uniform light intensity of the whole OLED array, it is preferable to drive the OLED panel by constant current.
FIG. 8 is a system architecture showing the conventional constant current driven OLED display panel and the driver. As shown, the driver includes a column driving circuit 81 and a row driving circuit 82. The column driving circuit 81 includes a reference bias generator 811 and a plurality of constant current column driver cells 812. FIG. 9 is a detailed circuit diagram of the column driving circuit 81. The reference bias generator 811 is coupled to each constant current column driver cell 812 to form a current mirror, so as to turn on the switch transistor MPS based on an input from a column data shift register 83 via an input terminal COLI, thereby an output transistor MPO providing a constant current output on the output terminal COLO. Furthermore, a discharge transistor MND, controlled by a discharge control terminal DIS, is provided in each constant current column driver cell 812 for eliminating the possible residual image caused by the junction capacitance and the wiring stray capacitance of OLEDs. The discharge transistor MND is turned on for a short period of time before the driving current is applied, so as to leak out the charge stored in the junction capacitors and the wiring stray capacitors of OLEDs.
With reference to FIG. 8 again, the row driving circuit 82 includes a plurality of inverters 821 connected to a row scanning shift register 84. Hence, under the control of the synchronous signals (HSYNC and VSYNC) and clock signal (HCLK), current from the output terminal COLO of a selected constant current column driver cell 812 is outputted to the OLEDs of a corresponding column. Furthermore, a selected inverter 821 drains the conducting current of a row of OLEDs, so as to turn on the desired OLEDs to emit light.
In a typical application, only dozens of micro amperes (e.g., 25 .mu.A) of driving current is sufficient for driving a pixel having a size of 0.1 mm.sup.2 to emit a required light intensity under a 1/64 duty cycle operating condition. However, taking a 64.times.64 OLED display panel as an example, a parasitic capacitance of several hundreds pico farads (e.g., 600 pF) may be generated from the stray capacitor on the thin film electrode layout and the junction capacitance of the diode array in driving each pixel. Therefore, if the constant current driving circuit as shown in FIG. 8 is employed for driving, the parasitic capacitor is charged by the driving current at first. As shown in FIG. 10, in a driving duration of about 200 micro seconds (.mu.s), it takes about 150 .mu.s to charge the OLED to have an enough voltage (e.g., about 7V) for conducting a current of about 25 .mu.A at the junction. Therefore, the actual duration for emitting light is greatly reduced, and the intensity of emitting light is not satisfactory.
To eliminate such a problem, a pre-charge capability is provided in the constant current driving circuit. A known driver with pre-charge circuit is shown in FIG. 11, wherein the gate of a PMOS transistor MPPRE, which is used as a pre-charge device, is temporarily grounded at the front edge of a driving period by a switch, so as to generate a large current in a short period of time rapidly charging a stray capacitor to a high voltage. However, such a design suffers from several disadvantages. With reference to FIG. 12, the first disadvantage is that the voltage of stray capacitor may be over-charged, resulting in a much larger junction current generated in OLED as compared to the predetermined driving current at this time period. The second disadvantage is that the over-charged voltage of the stray capacitor may be slowly discharged through OLED after the pre-charge process, resulting in a junction current being difficult to control. Particularly, the pre-charge process may produce a product of large current and time, i.e., a considerable amount of constant charge. As a result, it is difficult to adjust the driving current for obtaining a desired intensity of display panel. The third disadvantage is that an independent pre-charge control pulse signal with a very small width is required for alleviating the problem of uneven light emission caused by the first disadvantage. In view of above, the conventional constant current OLED drivers are not satisfactory, and thus there is a need to have an improved constant current driver to mitigate and/or obviate the aforementioned problems.